crostini: select the correct action button

A recent CL [1] added a second button with the class "action-button" to
the crostini installer. The existing logic assumed there is only one
"action-button" which, being no longer always true, sometimes results in
the wrong button being focussed/clicked. This CL updates the logic to
select the appropriate button, either next or install, depending on the
current state of the installation ui.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2004185

Bug: 1016195
Change-Id: I94cbba0f7610d9f444ee539ddcc910959ee817c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024232Reviewed-by: Julian Watson <juwa@google.com>
Commit-Queue: Jason Lin <lxj@google.com>
Auto-Submit: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#737675}

chrome/browser/resources/chromeos/crostini_installer/app.js

@@ -149,7 +149,7 @@ Polymer({
     });
 
     BrowserProxy.getInstance().handler.requestAmountOfFreeDiskSpace();
-    this.$$('.action-button').focus();
+    this.$$('.action-button:not([hidden])').focus();
   },
 
   /** @override */

chrome/browser/ui/webui/chromeos/crostini_installer/crostini_installer_ui.cc

@@ -150,9 +150,12 @@ bool CrostiniInstallerUI::can_close() {
 }
 
 void CrostiniInstallerUI::ClickInstallForTesting() {
+  // TODO(lxj): This assumes the config page does not show. When flag
+  // CrostiniUsername or CrostiniDiskResizing is turned on by default, we should
+  // click the "next" button first.
   web_ui()->GetWebContents()->GetMainFrame()->ExecuteJavaScriptForTests(
       base::ASCIIToUTF16("document.querySelector('crostini-installer-app')"
-                         ".$$('.action-button').click()"),
+                         ".$.install.click()"),
       base::NullCallback());
 }